Generally, particulates (particulate matter) contained in exhaust gas from a diesel engine are mainly constituted by carbonic soot and a soluble organic fraction (SOF) of high-boiling hydrocarbon and contain a trace of sulfate (misty sulfuric acid fraction). The particulates are captured by a particulate filter incorporated in an exhaust pipe through which exhaust gas flows.
The particulate filter has a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages; alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets have plugged outlets.
Only the exhaust gas passing through thin porous walls compartmentalizing the respective passages is discharged downstream.
The particulates in the exhaust gas, which are captured and accumulated on inner surfaces of the thin porous walls, require to be burned off for regeneration of the particulate filter before exhaust resistance increases due to clogging. However, the exhaust from the diesel engine in a normal operation status rarely has a chance to reach a temperature level at which the particulates ignite by themselves.
In order to overcome this, an oxidation catalyst comprising, for example, alumina carrying platinum and added with rare earth element such as cerium is integrally carried by a particulate filter. The oxidation catalyst facilitates an oxidation reaction of particulates captured by the particulate filter to lower the ignition temperature, so that the particulates can be burned off even at an exhaust temperature level lower than ever before.
However, even in such a case, a captured amount of the particulates may exceed a treated amount in an operation area having a lower exhaust temperature level. Continued operation at such lower exhaust temperature level may hinder sufficient regeneration of the particulate filter, resulting in excessive accumulation of the captured particulates in the particulate filter. Thus, when an amount of accumulated particulates has increased, it is necessary to forcibly heat the particulate filter to burn off the captured particulates.
More specifically, it has been conceived that a flow-through type oxidation catalyst is arranged in front of the particulate filter and a fuel addition valve is incorporated in the exhaust pipe upstream of the oxidation catalyst and that fuel added by the fuel addition valve is caused to make oxidization reaction through the oxidation catalyst and the exhaust gas elevated in temperature by resultant reaction heat is guided to the particulate filter to increase a catalyst bed temperature to thereby burn off the particulates, resulting in regeneration of the particulate filter.
There exists, for example, Patent Literature 1 showing general state of the art pertinent to this kind of forced regeneration of a particulate filter.